1. Field of the Invention
The present invention relates to an optical information recording-reproduction apparatus, and in particular, to an optical information recording-reproduction apparatus which records information on a recording medium on which information is recorded using holography and reproduces information from the recording medium on which information is recorded.
2. Description of the Related Art
When holography is used to record information, light with image information and reference light are superposed inside a recording medium and interference fringes produced thereby are recorded on the recording medium. When the recorded information is reproduced, the recording medium is radiated with the reference light to reproduce image information by diffraction attributable to the interference fringes. A holographic memory has drawn attention as a super high-density data storage in a practical application area in recent years.
Attention is attracted particularly by an optical disc memory in which image information is developed into two dimensional digital pattern information, then the information is recorded on disc-like recording medium such as CD and DVD using holography and reproduced from the recording medium.
The following literature introduces a recording-reproduction apparatus using a collinear holographic memory as such a technique: “Holographic Memory/Measurement & Nano Control Technologies for Brlostering HVD™”, Proceedings of 35th Meeting on Lightwave Sensing Technology, June 2005, p. 75-82, and “Holographic medium soon taking off to realize 200G bytes in 2006,” NIKKEI ELECTRONICS, Jan. 17, 2005, pp. 105 to 114.
The system is characterized in that information light and reference light are produced by the same spatial light modulator, transmitted along the same optical axis and converged on the recording medium with an objective lens to record hologram thereon. The system is also characterized in that producing only the reference light by the spatial light modulator and converging it on the recording medium enable information light to be reproduced by diffraction from the recorded hologram.
In such an optical disc memory, focus servo and tracking servo to radial runout of the tracks are normally conducted as is the case with CD and DVD.
Furthermore, the above optical disc memory requires a very high output light source such as a Q-switched laser to record information on a rotating recording medium in a short time. When a light source whose output is on the order of that of a typical semiconductor laser is used, a position radiated with light requires to be changed according to rotation to secure energy supplied for recording.
As a device in which a position radiated with light follows up rotation as described above, for example, Japanese Patent Application Laid-Open No. 2005-032306 discloses a device in which a collimator for a recording light source is moved perpendicularly to the optical axis to cause a position radiated with light to follow-up rotation.
FIG. 7 shows a conventional optical information recording-reproduction apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-032306.
Light from a recording and reproducing light source 1 passes through a first collimator lens 2, a polarization beam splitter 4, a spatial light modulator (information representing means) 35, a pair of relay lenses 6 and 8, a dichroic mirror 10, a ¼ wavelength plate 38, an objective lens 11 and reaches a recording medium 12. On the other hand, light from a servo reading device 13 advances into a second collimator lens 42 and the dichroic mirror 10. A photo detector 5 receives the reproduced light to reproduce the recorded information.
The servo reading device 13 detects positional information of the recording medium 12 and causes driving means 44 to move the first collimator lens 2 while following-up the movement of the recording medium 12. This is to record information at and reproduce information from a normal position while following-up the movement of the recording medium 12 by changing the angle of recording light incident on the polarization beam splitter 4 to shift the position of light incident on the recording medium 12.
Japanese Patent Application Laid-Open No. 2004-022083 discloses a device in which two galvano mirrors disposed in an optical system are used to perform tracking servo and rotation follow-up of a position radiated with light orthogonal thereto.
The above conventional examples, however, involve the following problems.
In general, a moving speed in a rotating direction at a position radiated with light changes according to radial runout even at a constant-speed rotation while a recording medium with eccentricity is rotated.
On the other hand, in conventional examples in which the above recording light source collimator is moved perpendicularly to the optical axis to cause the position radiated with light to perform rotation follow-up, light emitted from a servo reading device merges into a place nearer to the recording medium than the recording light source collimator.
For this reason, in the above conventional examples, the movement of the recording light source collimator does not exert any effect on light emitted from the servo reading device, which does not enable detecting change in moving speed according to radial runout, causing a problem in that accuracy in rotation follow-up is degraded.
In the above conventional examples in which two galvano mirrors disposed in an optical system are used to perform tracking servo and rotation follow-up of a position radiated with light orthogonal thereto, a mirror existing near the recording medium requires rotating. This causes a problem in that an apparatus is increased in size in the vicinity of the recording medium because positional interference needs to avoid against driving means for focus servo.
Alternatively, another mirror device requires providing in the optical system, which causes a problem with the restriction of optical design.